Technical Field
The present invention relates to a telescopic boom extension device for extending and retracting a telescopic boom mounted on a mobile crane.
Related Art
Mobile cranes such as a rough-terrain crane, for example, generally include a multistage telescopic boom. The telescopic boom is extended and retracted using a hydraulic cylinder in general. In particular, devices for extending and retracting a telescopic boom using a single double-acting hydraulic cylinder have been proposed (hereinafter, referred to as “extension device”) (for example, refer to JP 7-267584 A, JP No. 4612144, and JP No. 4709415).
The extension device is structured as described below.
The multistage telescopic boom includes bottom-stage and top-stage booms so called the base boom and the top boom, respectively, and one or more booms placed between the foregoing booms, which are so called intermediate booms. When the telescopic boom includes a plurality of intermediate booms, the intermediate boom adjacent to the top boom is referred to as a first intermediate boom, and the other intermediate boom adjacent to the first mediate boom is referred to as a second intermediate boom while the other intermediate boom adjacent to the second intermediate boom is referred to as a third intermediate boom, and so forth. Each of the booms extends (slides forth) and retracts (slides back) relative to the adjacent boom and is kept by a boom fixing pin (hereinafter, referred to as “B pin”) in the fully-retracted state and the fully-extended state. The B pins are provided between all the adjacent booms, and each of the B pins is elastically biased so as to fit to a corresponding one of the adjacent booms by a mechanical element such as a spring.
In the telescopic boom, the top boom is extended first, sequentially followed by the intermediate booms. One end portion (cylinder rod-side end portion) of the single hydraulic cylinder is coupled to the base end portion of the base boom. As described above, when the booms are in the fully-retracted state, the adjacent booms are coupled together always by the B pins. In this state, the cylinder tube of the hydraulic cylinder is coupled to the top boom. Specifically, the two are coupled together by a cylinder fixing pin (hereinafter, referred to as “C pin”). Unlike the B pins, the C pin is singly provided at the cylinder tube side, and is coupled to the top boom when the telescopic boom is in the fully-retracted state. As with the B pins, the C pin is elastically biased toward the telescopic boom by a mechanical element such as a spring such that the C pin always engages with the telescopic boom.
When the B pin coupling the top boom and the first intermediate boom is driven to decouple the two booms, the top boom becomes slidable relative to the first intermediate boom. When the hydraulic cylinder extends in this state, the top boom extends together with the cylinder tube relative to the first intermediate boom.
When the top boom enters in the fully-extended state relative to the first intermediate boom, the driving of the B pin is stopped and the top boom and the first intermediate boom are coupled together again by the B pin. Then, when the C pin coupling the top boom and the hydraulic cylinder is driven to decouple the boom and cylinder, the hydraulic cylinder is retracted. At that time, only the cylinder tube slides. When the cylinder tube moves to a predetermined position relative to the first intermediate boom, the driving of the C pin is stopped, and the C pin engages with the first intermediate boom to couple the cylinder tube and the first intermediate boom. Subsequently, the B pin coupling the first intermediate boom and the second intermediate boom is driven to decouple the two booms, and the hydraulic cylinder is extended. Accordingly, the second intermediate boom extends relative to the third intermediate boom. In this manner, each of the booms extends sequentially relative to the adjacent boom, and the entire telescopic boom is finally in the fully-extended state. In the reversed manner, the telescopic boom is retracted.
The B pins and the C pin are driven by the hydraulic actuator placed inside the boom. However, the B pins and the C pin are configured not to be driven all the time but to be driven only when coupling using the B pins and the C pin is to be cancelled as described above. That is, the hydraulic actuator is configured not to operate all the time but to operate temporarily only when the B pins and the C pin are to be removed for extension and retraction of the telescopic boom. Therefore, the flow amount of operating oil necessary for driving the B pins and the C pin is small. For this reason, according to the related art, an accumulator is e used as a drive source for the B pins and the C pin in some cases (for example, refer to JP 2013-539525 A).